Optical Coherence Tomography (OCT) is an imaging modality that is known to be efficacious for use in imaging retinal tissue. In general, OCT imaging is analogous to the more well-known technique of ultrasound imaging. Unlike ultrasound imaging, however, OCT imaging uses light instead of sound. Importantly, OCT imaging can be accomplished in situ, and in real time. Like almost every other optical measuring technique, in order to be effective, OCT imaging requires a useable signal-to-noise ratio (SNR). Stated differently, optical aberrations that may be introduced into an imaging light beam need to be eliminated or significantly reduced before the beam can be most effectively used for imaging.
The ability of a system to image an object will depend on the nature of the particular application and, most importantly, the physical characteristics of the imaging light beam. With this in mind, light returning from inside the eye can be generally categorized as being either backreflected (i.e. regular reflection of light), or backscattered (i.e. irregular reflection or dispersal of light). Importantly, these categories of light can be analyzed in different ways, for different purposes. And, depending on the purpose (i.e. application), backreflected and backscattered light can be evaluated differently in either the time domain or in the frequency domain.
In a time domain analysis, a beam of light that is backreflected from a target tissue can be evaluated using conventional wavefront analysis techniques. Also, in a time domain analysis, OCT techniques can be employed when an interferometer is used to identify the wavelengths of light that is backscattered from a target tissue. Typically, these time domain techniques will be accomplished using a Hartmann-Shack sensor. In these time domain analyses, evaluations can be performed to detect aberrations that are introduced into an imaging beam by the anatomical structures that are in its path. For example, it is known that anterior components of an eye (e.g. the cornea and lens) will introduce anterior optical aberrations into a light beam that passes through the components. Insofar as the retina is specifically concerned, it is also known that phase aberrations are introduced into a light beam as it passes through retinal tissue. Fortunately, these introduced aberrations can be measured.
In the Fourier domain (i.e. frequency domain), OCT techniques can again be used on backscattered light. This time, however, rather than using an interferometer and a Hartmann-Shack sensor for wavefront analysis as is done in a time domain analysis; in the Fourier domain, OCT techniques typically use a spectrometer that evaluates frequency distributions in the light beam. Further, instead of measuring aberrations, the purpose for using the OCT techniques in the Fourier domain involves imaging. As indicated above, this is preferably done with as high of an SNR as is possible.
In light of the above it is an object of the present invention to provide a system and method for imaging a tissue cell at a predetermined depth in the retina of an eye, with compensation for refractive errors. Another object of the present invention is to provide a system and method for imaging a retinal tissue cell wherein the signal-to-noise ratio (SNR) is sufficiently high to allow for ultra-high-resolution OCT. Still another object of the present invention is to provide a system and method for imaging retinal cell tissue that is easy to use, is simple to implement and is comparatively cost effective.